There are a variety of ways to achieve the crosslinking of polyolefins such as polyethylene. The most common way is through the use of peroxide crosslinking agents added to the resin. The peroxides provide a source of free radicals when heated above their decomposition temperatures. These free radicals are capable of extracting a hydrogen from the polyolefin backbone thus transferring the free radical site to the polyolefin. With this accomplished, two polyolefin chains can crosslink together. When this is carried out through the matrix of the resin, the molecules become tied together with covalent bonds and a crosslinked network is formed.
Peroxide crosslinking provides molded articles with excellent high temperature properties, but care must be taken during the thermoforming process. For example, extrusion temperatures must be kept quite low. If extrusion temperatures are above the peroxide decomposition temperatures, premature crosslinking (scorch) may occur. This temperature restriction limits the rates at which peroxide curable polyolefin can be extruded.
A second method of crosslinking polyolefin is through the use of irradiation. In this case, the free radical formed on the polyolefin backbone is the result of electron beam irradiation. This technique overcomes the extrusion restriction for the peroxide crosslinked systems noted above, but has restrictions of its own. Specifically, thick sections of insulation become difficult to cure uniformly and products with non uniform cross sections pose a challenging engineering problem. In addition, high energy irradiation equipment is expensive and a significant amount of safety shielding is required.
In the above two methods of crosslinking, carbon-carbon bonds are formed at the crosslink sites. This is in comparison to the siloxane bonds which form in the third type of crosslinking--moisture cure.
Moisture cure involves the crosslinking of silane modified polyolefins. The technique is dependent on modifying the polyolefins backbone with a silyl trialkoxy moiety, preferably where the alkoxy is methoxy or ethoxy. The modified polyolefin will only crosslink in the presence of water. In practice, the resin has catalyst incorporated in it to speed up the crosslinking reaction. By excluding moisture, high temperature extrusions are possible and the material can still be processed as a thermoplastic. This allows for high line rates. After the extrusion, a separate curing step is conducted by placing the extruded system into a water bath or sauna. Usually the water bath is at an elevated temperature (70.degree.-95.degree. C.).
There are three routes for producing the silane modified polyolefin, all of them involving the same vinyl trialkoxysilane (VTAS), but they differ in the time sequence, the complexity and the procedure for adding the silane. In one such procedure, the silane is incorporated during the reaction with an olefin to make a polyolefin addition copolymer. The resin goes directly from a reactor to the fabricator's extruder without any grafting in compounding equipment. This ensures a high degree of cleanliness and excellent control of the density and molecular weight distribution of the product. The chemical structure of the produced compound insures that the product will have at least a two year shelf stability. In order to crosslink the silane copolymer most effectively, a catalyst such as a tin catalyst is required. This is usually supplied in the form of a catalyst masterbatch prepared in a separate compounding step. For optimum performance, the catalyst masterbatch must be dried prior to use.
In a second procedure for the production of modified polyolefin, peroxide-grafting of VTAS to polyolefin is accomplished. To accomplish this, a peroxide is mixed with the silane and the polyolefin and all these components are compounded at high temperatures. During this compounding step, grafting of the VTAS occurs. In addition, some peroxide crosslinking occurs. Producers of these "Sioplas" type products must start the compounding with a polyolefin which has a melt index of about 10. After the compounding, the grafted product has a melt index of the order of one. The drop is due to the partial crosslinking (undesirable) of the peroxide acting on the polyolefin. These products have the potential disadvantages of the presence of unreacted silane and peroxide and of difficulty in controlling the grafting step, yielding a variable final melt index. Furthermore, the specific chemical structure of the graft copolymer yields a product which only has about a six month shelf stability. Similar to the thermoplastic olefin silane copolymer produced by the first technique, the fabricator of "Sioplas" resins would blend the resin with a dried catalyst masterbatch in the extruder, process the system as a thermoplastic and cure the product off-line in a water bath.
The third route for producing the silane modified polyolefin is through what is commonly called the Monosil/BICC process (see for example U.S. Pat. No. 4,117,195). In this case, a polyolefin, normally a polyethylene, a vinyl silane, a tin catalyst and a peroxide are all mixed together in an extruder/reactor at the fabricator's plant. The extruder utilizes a long extruder screw with a L/D of about 30:1. This enables the components to be mixed and reacted (grafted) during the extrusion process. As in the case with the Sioplas technology, a significant drop in melt index occurs. Great care is needed to achieve the grafting without excessive crosslinking and some unreacted silane and peroxide may pass through the system and remain in the resin. The specialized extruders are more expensive than general purpose polyethylene extruders used for thermoplastic olefin silane copolymers.
A feature common to all these processes is that in the fabrication step the silane modified resin is being processed in the presence of the silanol condensation catalyst, generally a tin catalyst. Under this condition premature crosslinking, scorch, through condensation of silane moieties may occur thereby leading to changes in melt viscosity and extrusion instability. In those cases when scorch becomes severe the process must be terminated and the equipment cleaned to removed scorched polymer before beginning again. This results in high scarp production and costly equipment outage adding to the overall cost and complexity of operation.
The present invention obviates these problems by allowing the fabrication process to occur in the absence of the condensation catalyst but permitting rapid and essentially complete crosslinking through application of the invention disclosed herein.
The present invention is primarily directed to an improvement in the crosslinking process for thermoplastic olefin silane copolymers.
It is noted that in the conventional process, the following sequences are evident.
1. The olefin silane copolymers are in contact with a catalyst such as a tin catalyst in an extruder, where unfortunately scorching can take place;
2. Catalyst masterbatch is prepared in a separate compounding step and drying before use is generally required;
3. The curing step is conducted by placing the extruded article in a steam or water bath at high temperatures.
In order to promote curing or achieve a faster curing rate and control of scorch, a carboxylic acid, such as acetic, formic, propionic, butyoic, benzoic and like acids can be compounded with the olefin silane copolymer of step 1 (see for example U.S. Pat. No. 5,047,476 issued Sep. 10, 1991 assigned to a common assignee and U.S. Pat. No. 4,680,319 issued to Gimpel).